How to control the thickness and darkness of lines when exporting UV Layout
I had a similar issue with the edge being 4px. There are a couple of options but none I can see to fix this. Preferences > theme > UV Image editor. You can change the vertex size but not the edge. Also on the overlays there are no specific size options in the Display As



Instead I exported the UV Map as .svg or .eps from here you can open in Adobe Illustrator which gives a sharp vector path. I would set the fill opacity to 0 to make sure there is no bleed from the black edge. You could open either of these formats in Ps but the same issue persists.



In Illustrator you can then adjust the stroke width from 1pt to 0.1pt and copy into Photoshop to give a single pixel width. Not perfect but the best I can see unless there is another option I am missing in Blender